VLAN Discovery Process
An option that we have in our network is to set up multiple VLANs. We do this for many reasons. To separate out traffic, and then we also get some security because of that. So, by separating out your data traffic and your voice traffic, we do have some security in place. Also Quality of Service; being able to mark traffic because it's in a certain VLAN, like your voice traffic, makes it much easier to implement. Our IP phones have a built-in switch port and it provides dedicated connection to your phone, your PC, and then we also have Port 2 that will then connect up to the access switch, or whatever device we're plugged in to, and that is the port that we get the inline power from. So port 0 is an internal Ethernet interface that carries IP phone traffic. Port 1 actually connects to the PC or whatever you plug into it. And then Port 2 connects back to the switch that then provides the online power, if you are using that. And within our voice VLANs, remember we have several different types of VLANs and different types of traffic that could be running down those VLANs.
Our IP phones use CDP, again to learn the VLAN configuration from your switches. And if the CDP protocol is enabled on the switch port, the switch instructs the phone to treat the Layer 2 class of service priority value of that attached PC in one of the following ways. Remember we talked about Quality of Service, and the phone is going to go ahead and label its traffic with a class of service Layer 2 value of five, that's the highest. Six and seven are reserved for maintenance and other traffic in the network.
Our PC on the other hand, we don't want necessarily to have that high of a value. So we can do one of two things. We can trust it and the IP phone allows the PC to send its own frames with whatever class of service value it has. We can not trust it or set It up in an untrusted mode, which is the default, where the IP phones says "I don't care" to the PC, "what you send your priority value as, I'm giving you a value of zero". Or we can configure what that class of service priority level is. So, we have three different options, the default is the phone doesn't trust the PC or whatever is plugged into it and it just marks it as a class of service of zero.
IP Phone DHCP Configuration
Now DHCP works similarly for your IP phones as it would for handing out an IP address to your laptop or your PC plugged into the network. So the phone powers on and it initializes the IP stack and sends a DHCP request. "Is anybody out there that can give me an address?" The DHCP request is sent as a broadcast to everybody in the voice VLAN. Now all the devices in the voice VLAN receive it, but the only one that responds to it is the DHCP server. So everybody gets it, but most people ignore it until we hit that DHCP server. It then response with an offer. In that offer you can get an IP address, a subnet mask, a default gateway and the very, very important TFTP server address, which we fill in as option 150. That's the option parameters that says, "okay here is the IP address of your TFTP server". And once that happens now, the phone has its address and now knows the location of that TFTP server.
DHCP Server Feature Support Overview
Now where is your DHCP server? I should say, who is your DHCP server? You could make your Communications Manager your DHCP server, but it's only recommended for your IP phones. You could leverage other environments to hand those addresses out, but for a smaller deployment up to about a 1,000 IP phones it'll work. Only one DHCP server is recommended for your Communications Manager cluster and DHCP is a standalone server, there's no backup server that exists. The DHCP server is normally provided on the publisher and we could have multiple subnets configured on it. But my strongly recommendation and I'm always doing it, I use a different device to be my DHCP server. You know who would work really good for your IP phones? A router. You can make the router a DHCP server. Again, I wouldn't use it from my PCs probably, but I would consider it for my IP phones. We have a lot of configuration options out there, including option 150 for the TFTP server information, so if you're not comfortable having your Communications Manager hand out DHCP information you might want to consider the router.
TFTP Device Configuration XML File
Now let's pretend that your IP phone is brand new, out of the box and you're plugging it in for the first time. What is the expected behaviour? It's going to take a little bit longer to boot up, it's going to go through several iterations, you will see it on screen. Because what has to happen is the IP phone is going to look for a specific file from the TFTP server, and that specific file contains the MAC address. But because we've got this brand spanking new phone that doesn't have a TFTP server file just yet, we have to download the XMLDefault.cnf.xml file. Now the IP phone obtains its list of Communications Managers, and it then attempts to auto-register to the primary server. And in that process that's where you're going to see things can slowdown, just a bit. Because it's going to try to register with the Communications Manager. It then has to create its unique file, because that unique file is going to be what we're going to leverage every time that phone reboots. And that's going to make things happen quicker too, but it's going to return information including the MAC address as part of that config file.
So, the IP phone with this MAC address, downloads this configuration file that it now has, specific to that phone and it's going to contain the information for that IP phone to register with its Communications Manager. And again it's going to see IP addresses here. So these IP addresses will give them a direct connection back to the Communications Manager server that is going to be hosting that IP phone for you.
DHCP on Cisco Unified Communications Manager Express
Now on the Communications Manager Express, it is a router. So I have no qualms saying go out to the Communications Manager Express device and configure your DHCP information and you can see the following example.
ip dhcp pool IPPhones
network 192.168.10.0 255.255.255.0
option 150 ip 192.168.10.240
default router 192.168.10.1
So, I really think it's kind of simple to go out to the device and set up your DHCP configuration, because this is very similar to what you would do on another router if you were using Communications Manager to set up your DHCP server.